As more research and development is done in the construction industry, methods of constructing buildings and their respective materials have been changing. This is especially true for building envelopes that protect the interior of buildings from the external environment.
As a direct result of these developments, the building envelopes of older homes do not function the same way that the homes we build today do.
The Challenges of Older Homes
Older homes have limited insulation in the wall and attic cavities, as well as no insulation in the basement. They commonly feature single pane windows, a forced air furnace and a humidifier. These types of home building envelopes very commonly dry faster then the assemblies do today. Here’s why:
Forced air furnaces draw air in and heat it, causing the air to expand and draw in moisture. As the hot, moist air is pushed out of the chimney or external air exchange, negative pressure is created. This build-up of negative pressure not only draws moisture out of the building but also encourages air transport across the building assembly, drying out the envelope in the process.
This type of building layout tends to have drawbacks for residents as well; The colder air being sucked into the home makes areas near walls drafty and cools the house down when the furnace was trying to heat it, meaning a lack of energy efficiency is present. As a result of these complications, we’ve seen the shift to what we now know as standard practices for new home construction.
Newer homes are constructed with better insulation, improved airtight construction practices, different and often higher performing materials within the building envelope, and sealed combustion furnace systems. These furnace systems draw air from the exterior of the home, and output the exhaust to the exterior of the home, resulting in no pressure difference between the interior and the exterior of the home. In combination with the more airtight construction practices, it is rare that exterior air can travel to the interior of a home. This reduces the uncomfortable drafts, reduces the need for humidifiers in the home because the moisture isn’t being sucked out of the home, and drastically improves the energy efficiency.
Retrofits and installation
Installing these new systems and materials in older homes can be tricky, as the envelope drying and draining mechanisms do not operate in the same ways. The major danger of upgrading an older home to be more energy efficient is the wall and attic assemblies will not have the ability to dry in the way that they use to. Installing a sealed combustion furnace in an older home takes away that driving pressure difference within the home, and the airtight construction allows for even less air to travel through the building envelope assemblies. If installed properly with the proper considerations, you could reduce your energy costs by up to 50% increase the humidity in your home (lack of humidity can cause health problems), and increase the comfort of occupants in the home.
It is important to note that when adding extra insulation on to an existing home, consideration for vapour barriers and how they function can be lacking. The extra layer of vapour barrier may result in 2 vapour barriers in the building envelope, depending on the existing materials, and very commonly ends up on the cold side of the building envelope because the extra insulation installed on top of it is not enough to keep it the same temperature as the interior of the home. When the warm air from the interior of the home comes into contact with the second vapour barrier it can no longer travel through the envelope and upon coming into contact with a cold surface, will often produce condensation. Now that the building assembly has two vapour barriers, it’s ability to dry is significantly reduced. Slowly overtime the moisture that is not able to dry out of the building envelope will accumulate overtime and eventually rot the materials surrounding it such as the structural framing and sheathing. This is a very slow process, and can take up to tens of years before signs of failure can occur.
It is also impractical in many cases to simply add a layer of continuous insulation to the exterior of a building, as re-applying trim around penetrations such as windows and doors can be costly and a hassle. Also in the case of heritage homes, this may cover up many of the desirable architectural features that make these homes so unique. For more on rehabilitation of masonry heritage homes click here.
To avoid these problems, many homeowners and builders alike opt to use spray foam insulation for these upgrades, as it provides the highest increase in R-value for minimal thickness addition. This foam however is vapour permeable, and as explained above if not installed in the right location of the building envelope and around the right materials, can cause serious problems and damage in the future.
The best way to prevent this is to have an envelope engineer analyze the proposed renovations and additions to the home to ensure that either condensation will not form on the second vapour barrier and to guarantee that there is a way for the water to exit the building envelope should it start to accumulate.
To learn more about retrofits and renovating older buildings, contact us today!